Most of the commercially available liquid crystal displays (LCD) make use of the birefringence of the liquid crystal material and need one or two polarizers to convert the polarization state of the transmitted light into a brightness level. On the other hand, there are LCDs, which are based on scattering of light, and which do not require polarizing sheets. The scattering liquid crystal device is a device that switches from a transparent state to a white turbid state by applying a voltage, thus controlling light transmittance. Because there is no absorption by a polarizing plate such a device offers higher light transmittance. Because of these properties, scattering type liquid crystal devices are also used as switchable windows, which can convert the window from being transmissive to an opaque state. There are two types of scattering liquid crystal devices: the first is the normal type, which is in a scattering state when no voltage is applied and changes to the transparent state upon applying a voltage. The second is a reverse type which is transparent without applied voltage and which changes to the scattering state when a voltage is applied. The normal type is usually used in displays for mobile phones or for transparent-opaque switching of glass windows. For use in a segment type display device, typically the whole display is in a scattering state and an electrode portion becomes transparent upon applying a voltage. For use in a projection display, the reverse type is preferred, such that the display surface is transparent and the electrode portion changes to a scattering state upon applying a voltage.
Most of the scattering type liquid crystal devices are based on a composite material in which a liquid crystal material is dispersed in a polymer. The name “polymer dispersed liquid crystal” (PDLC) is broadly used for the technology and the related devices.
EP0488116A2 describes a constitution of a reverse type polymer dispersion liquid crystal device, which is transparent when no electric field is applied. It uses a liquid crystal polymer composite layer in which a polymer and a liquid crystal are mutually dispersed and both are oriented in the same direction when no electric field is applied. Alignment of the liquid crystals may be parallel or perpendicular to the substrate. If the alignment is parallel to the substrate, liquid crystals with positive dielectric anisotropy are being used, whereas liquid crystals with negative dielectric anisotropy are used in case of vertical alignment (VA). In order to align the liquid crystals, alignment treatment of the substrate surface may be performed, for example by depositing an alignment layer with parallel or perpendicular alignment properties, respectively. In case of parallel alignment, subsequent brushing of the alignment layer defines the orientation direction within the substrate plane, whereas in case of perpendicular alignment no further alignment treatment is required. In the examples, the ratio of polymer to liquid crystals is approximately 1:10 or 15:85. EP0488116A2 teaches that the optimum amount of the liquid crystal is 50-97% and that the proper contrast will not be yielded if the liquid crystal content is higher than 97%. If the composition comprises a polymer precursor, the pre-polymer is polymerized by exposure to uv-light at room temperature.
A drawback of many PDLC devices is a difference of the voltage-transmission curves measured with increasing and decreasing voltage, which is referred to as a hysteresis.
H. Murai et. al., J. Appl. Phys. 81(4), p. 1962, disclose a homeotropic reverse-mode polymer-liquid crystal device, which was investigated for a polymer content of 1 to 5 wt %. The polymer has been formed by initiating the monomer reactions in the liquid crystal phase, which results in a polymer network, which is aligned in the homeotropic direction. A monomer content of 3-5 wt % in the liquid crystal turned out to give good properties. If the content is less than 3 wt %, there is insufficient on-state scattering and often the transmittance does not return to the initial value when the applied voltage is turned off. In the experiments the cell substrates were covered with a homeotropic alignment layer, but no rubbing process was applied.
JP200034714 discloses a VA-LCD with a liquid crystal layer comprising a polymer dispersion. The liquid crystal layer has been made by providing a mixture of a liquid crystal and a monomer between a pair of substrates and polymerizing the monomer while the liquid crystal is in the liquid crystal phase. The polymer dispersion which is formed maintains the pretilt angle of the liquid crystal molecules in the liquid crystal layer.
U.S. Pat. No. 5,496,497 discloses a composition comprising a liquid crystal as well as a mono- and a bifunctional acrylate component, the latter two having a polarity within a certain range. A PDLC device made with such a composition shows reduced hysteresis.
Although one of the advantages of a polymer dispersed liquid crystal device is that it does not require polarizers to observe the brightness difference between on and off state, it still can be equipped with such polarizers, which has the advantage of higher contrast. In case of a normal type VA polymer dispersed LCD the contrast is particularly high. However, the hysteresis of state of the art VA polymer dispersed liquid crystal displays is too high to display grey scale images with high quality.